Scanning photoelectron spectromicroscopy experiments on the edge of patterned Si overlayers on GaSe revealed the presence of an inhomogeneous reaction with a metallic-like Ga phase separation. The Si-Se chemically shifted components show lateral variations and a thickness-related SiSe2/SiSex (x = 0.5, 1, and 1.5) ratio on a micrometer scale. The dependence of the peak intensities on the overlayer thickness suggests an initial layer-by-layer coverage until approximate to 2 Angstrom, followed by a clustering growth mode. (C) 1999 American Institute of Physics. [S0021-8979(99)07111-X].